The present invention pertains to systems that are used in calibrating flowmeters for purposes of assuring accuracy from the meter which is calibrated. More specifically, the system uses statistical analysis to calibrate volumetric flowmeters, mass flowmeters, densitometers, and viscosimeters in situations where the meter being calibrated can be of the same type as the standardized meter.
It is often desirable to perform periodic maintenance upon flow meters that are placed in service. One aspect of this maintenance is to calibrate the meters for the purpose of ensuring accurate and reliable measurement data. As used in the discussion below, the term xe2x80x9cstandard meterxe2x80x9d is hereby defined to mean a meter that has been calibrated according to precise standards where this calibration permits the meter to be used as a standard measurement tool for use in calibrating other meters. The term xe2x80x9cservice meterxe2x80x9d refers to a meter that is normally in use obtaining measurement data for a specified purpose, but periodically requires calibration to ensure the accuracy of this measurement data. A standard meter is also a service meter in the sense that the standard meter is normally in use obtaining measurement data for the purpose of calibrating other meters, and a standard meter itself requires periodic calibration.
The purpose of a meter calibration effort is to ascertain a flow calibration factor that is used to convert electronic signals to direct measurements of mass, volume, and other information from the meter under test. Coriolis meters and positive displacement meters are known in the art as linear meters, i.e., the flow calibration factor is a constant with respect to flow rate. Other meters including orifice meters, magnetic flow and vortex meters are nonlinear meters where the flow calibration factor varies with flow rate.
The calibration process typically entails removing the meter from service for shipment to a test facility where the meter is cleaned, repaired as needed, and subjected to test measurements. The most common calibration measurements usually involve the use of a gravimetric diverter system to flow through the meter under test a standard fluid having precisely known intrinsic and extrinsic fluid properties, e.g., temperature, density, viscosity, and volume. The meter under test performs flow measurements on this fluid, and these measurements are cross checked against the known fluid properties. Gravimetric diverter systems may be designed for testing purposes across a wide range of flow rates, but the additional structure that is required to provide this functionality is so large as to make transportation of these systems impracticable.
The use of gravimetric diverter systems to test flowmeters is relatively time consuming and expensive. The gravimetric diverter systems themselves occupy relatively large volumes of space. The loss of time, space and money can be reduced by calibrating very precise meters, i.e., standard meters, against gravimetric standards for subsequent use in calibrating other meters under test. During the course of calibration tests, these standard meters are connected in series with the meter under test to perform simultaneous flow measurements. The measurement data from the meter under test is used in calculations with measurement information from the standard meter on the same fluid volume to provide or confirm a flow calibration factor for the meter under test.
Flowmeters can never be relied upon to provide measurement data that is completely accurate because there are always small uncertainties in the meter output. For example, many Coriolis flowmeters sold by Micro Motion, Inc., of Boulder, Colo., are specified to be accurate within 0.1 percent of the total mass flow rate within a selected operating range of flow rates. Many of these meters are capable of even more outstanding accuracy down to less than 0.01 percent within subportions of this range. There is no one single flowmeter that provides this outstanding level of accuracy across all flow rates. Coriolis flowmeters have been designed to perform mass flow rate measurements on flows ranging from less than 0.1 lbs/min to greater than 25,000 lbs/min.
Coriolis effect mass flowmeters are well known and have been described in numerous patents, e.g., in U.S. Pat. Nos. 4,444,059, 4,491,025, and 4,422,338 to Smith, which all describe mass flow rate meters that use vibrating tubes to impart measurable Coriolis effects which are related to mass flow rate. U.S. Pat. No. 4,491,009 to Ruesch describes a vibrating tube densitometer based upon the structure of a Coriolis mass flowmeter. The ability of Coriolis effect mass flowmeters to measure density permits the determination of a volumetric flow rate by a simple division of the density value into the mass flow rate value. It is also well known that Coriolis effect mass flowmeters can be operated as viscosimeters.
The total level of uncertainty in a flow measurement arises from random uncertainties and systematic uncertainties in the meter together with its environment of use. The metering industry has generally considered these uncertainties and published official guidelines for quantifying and managing meter uncertainties, as in ISO-5168, which is hereby incorporated by reference to the same extent as though fully disclosed herein.
The metering industry uses a rule of thumb advantage that requires the uncertainty in output from a standard meter to be at least three times better that the manufacturer""s accuracy specification of a meter under test. Thus, a service meter that is specified as being accurate to 0.1 percent of a flow rate requires a standard meter that is accurate to 0.033 percent for calibration purposes.
Coriolis mass flow meters are the most accurate type of meter known for practical use through many flow regimes. The meters are generally insensitive to flow profile, and calibration factors that are developed using liquid fluids work equally well when applied to gas fluids in service. There is no known or readily available metering technology with superior accuracy for use as a measurement standard against Coriolis effect mass flow meters.
Some problems could arise when using Coriolis meters as the standard meters to calibrate Coriolis service meters. Where the two Coriolis meters have similar or identical manufacturer""s accuracy specifications, it becomes impossible to gain the rule of thumb advantage requiring uncertainty in the standard meter to be three times better than the manufacturer""s accuracy specification in the meter under test. This situation digresses into a requirement for relatively expensive gravimetric testing of the service meter.
There exists a need for a compact flowmeter calibration system including standard meters that are operable across a wide range of flow rates with sufficient accuracy for use as a standard metering system. The system should be operable for calibrating linear and nonlinear meters. The system should also be modular so it can be manufactured, shipped and installed easily. The system can also be transported for calibration of meters at remote sites where meters are in service, as opposed to the present practice of removing the meters from service and shipping the meters to a flow laboratory for calibration purposes.
The present invention overcomes the problems that are outlined above and advances the art by providing a compact flowmeter calibration system including a plurality of standard meters with sufficient accuracy for use as standard meters across a wide range of flow rates. The system is operable for calibrating both linear and nonlinear meters through a wide range of flow rates. The system is also easily transported to remote locations for calibration testing, and can be broken down into modular components for further ease of transportation and storage.
As used herein, the term xe2x80x9ccalibrationxe2x80x9d is defined to mean a flow measurement test that provides data which is used to either improve the accuracy of a flowmeter or to verify the accuracy of a flowmeter. Improvement of flowmeter accuracy is most often done by changing a flow calibration factor for the meter. The term xe2x80x9cflowmeterxe2x80x9d is defined to mean any meter having the ability to measure intrinsic or extrinsic fluid properties when placed in a service location where the fluid is normally flowing. Flowmeters include densitometers and viscosimeters, as well as mass flow rate and volumetric flow rate meters. Volumetric rate flowmeters are preferred for use in systems according to the present invention, and mass rate flowmeters are especially preferred. The term xe2x80x9cfluidxe2x80x9d is defined to include liquids; gasses; mixtures of liquids and gasses; mixtures of liquids and solids that primarily exhibit liquid behavior; mixtures of gasses and solids that primarily exhibit gas behavior; and mixtures of gasses, liquids, and solids that primarily exhibit liquid or gas behavior. The term xe2x80x9cuncertaintyxe2x80x9d means a combination of random and systematic uncertainties that is performed according to any convention that is accepted in the metering art, at least including international standards such as ISO-5168.
The flowmeter calibration system includes a mechanism for supplying fluid to use in flow calibration measurements. The system performs flow measurements upon this fluid in a succession of three steps. The first step is a quality check step that is performed using a first flowmeter array. The second step includes performing flow measurements using a service meter under test. The third step is a standard meter measurement that is performed using a second flowmeter array. A controller, based on statistical comparison analysis between the first and second arrays, directs the flow rate through the system to optimize the accuracy or sensitivity of measurements in both flowmeter arrays. The flow measurements are used to calculate or confirm a flow calibration factor for use in the meter under test. The same principles apply whether the meter under test is being calibrated for mass flow rate, volumetric flow rate, density, or viscosity measurements.
Preferred embodiments of the flowmeter calibration system incorporate a modular design for ease of transport and storage. The system can be separated into its respective subassemblies including the first flow meter array, the second flowmeter array, and the fluid supply mechanism. It is often the case that calibration can be performed using a preexisting fluid supply at the test site and, consequently, it is not always necessary to transport a fluid supply means with the test system.
The fluid supply mechanism can provide any fluid that is compatible with the system hardware; e.g., a liquid reservoir and a pump; a multiphase fluid including multiple immiscible liquid phases and gas; an attachment to a pressurized water supply, such as plant process fluids, a city water supply, artesian well, or gravimetric system; and a pressurized gas supply, such as natural gas, air, or plant process gasses. A constant pressure source of water is the preferred supply mechanism for use in calibrating Coriolis flowmeters.
The first flowmeter array and the second flowmeter array each contain at least one flowmeter. The first flowmeter array is operably configured to receive fluid from the fluid supply mechanism. A meter under test is positioned in the flow pathway between the first flowmeter array and the second flowmeter array. It is intended that all of the fluid passing between the first flowmeter array and the second flowmeter array must pass through the meter under test. A number of arrays may be connected in parallel to accommodate extremely large flow volumes.
It is much preferred that the first flowmeter array and the second flowmeter array each contain a plurality of flowmeters, and that these flowmeters are Coriolis effect mass flowmeters. The flowmeters of each array have different flow capacities and different flow ranges of optimal measurement sensitivity corresponding to relatively low uncertainty. Each meter in the first flowmeter array preferably has a substantially identical match in the second flowmeter array. The flowmeters in each array are configured to flow in parallel with respect to one another, as opposed to series flow within each array.
Each of the first flowmeter array, the second flowmeter array, and the meter under test provide flow measurement signals to a central CPU-based controller. The controller opens and closes automated valves leading to each meter for the purpose of adjusting flow through each meter into the range of optimal measurement sensitivity corresponding to relatively low uncertainty for each meter.
Prior to insertion of the meters in the arrays the meters are characterized against a traceable reference standard to determine the range over which the meter meets the uncertainty requirements. These characterizations are used by the controller to interpret the data from the meters in the arrays. These meter characteristics, in combination with the controlled ability to flow through optimal ranges of the standard meters, advantageously permits Coriolis technology meters to be used in the calibration of similar or identical Coriolis technology meters.
These aspects and advantages of the present invention are apparent based upon the discussion to follow. It is an aspect of the present invention to:
provide a standard metering system that is capable of calibrating both linear and nonlinear flowmeters;
provide calibration measurements across a wide range of flow rates ranging from less than 0.1 lb/min to 7000 lbs/min or even greater than 25,000 lbs/min;
achieve calibration of a Coriolis meter under test using Coriolis meters as the calibration standard in flow ranges where there exists no practical superior metering technology to Coriolis technology;
provide complete automation of the calibration process;
provide a compact system that can be used to calibrate flowmeters across a wide variety of flow rates while occupying less space than conventional diverter gravimetric systems;
provide a design that is optionally transportable on a small truck and can be easily assembled from modular components for ease of transportation and storage; and
calibrate volumetric meters using temperature, pressure, and mass based references.